Decent Compass for GEM setup?

[Stonius (Markus)]

I bought a cheapie orienteering compass online to help get a rough alignment prior to setting everything up so that the SCP fall within the range of adjustment fo the mount, but I've found it to be next to useless as the motors skew the needle. There is no parallel surface I can use that will not be too close to the motors and the magnets enclosed within. Even outside of any interference it's pretty crap, wandering all over the place, never seeming to settle definitively.

I don't know much about compasses and what to look for. Any recommendations for a decent one?

Best,

Markus

[bojan]

First, compass must be away from any ferrous objects and magnets.

Mount it on 1m arm (wood, aluminium), see here.
And, don't forget about correction, 11°~for Melbourne.

[Stonius (Markus)]

Ah, an arm - that would be a good idea! Yes, of course. You engineering types have an answer to everything :-) I was aware of the 11 degree correction, but I still would like to find a more responsive compass. I had no idea there were compasses and then there were *compasses. I mean, it's a magnetic stick in oil, for crying out loud! :-) Looks like yours was a DIY version, am I right?

Thanks for the pics. What is that precipice you have your scope set up next to? Is that a trap for nosy neighbours in the dark? :-P Knowing me it would be a trap for *me; "I can almost see vesta over that tree now aaaaah!"

Cheers mate :-)

Markus

[bojan]

Markus,
My compass (needle and gray-green housing) came from some military scrapyard, looong time ago ('70-ies?).. but everything else is DIY.
Precipice is retaining wall, less than 1m high, I just wanted tripod to be on stable surface (concrete pavement, positions for 2 legs are marked with chisel) and to have easy access to polar finder eyepiece.
The third leg is sitting in the hole of the 3/4" water pipe, hammered 25cm into ground (not visible on pictures because of the grass) - so every time I set it up, already have rough alignment.).
Cheers,

B

[Mamba (David)]

I am amazed to find that the compass on my I-phone is very accurate, certainly within 2 degrees
Dave Cooke

[OzEclipse (Joe Cali)]

Spyglass is an app for iPhone. It overlays the camera live video image with the compass direction corrected for magnetic declination.

Stepper motors and servo motors have strong permanent magnets. You need to keep any compass including a smartphone app electronic compass away from the mount. I stand about 2m behind the mount, move until the centre of the mount is between me and the point on the horizon due south which is visible on the phone/iPad camera image.

Lay the dec axis horizontal and providing your optical axis is parallel to your RA axis (no cone error) then you can use finder and prime optic to point the RA to the point on the south horizon.

Another method
If you always observe from exactly the same spot, drift align the mount precisely once. Level the dec axis and use the prime optic to spot or photograph the horizon point. Use the photo next time to point the RA.

If you want to use an orienteering compass, go to an outdoor store. Price is usually a good indication of quality. Brands like Suunto and Silva are the old favourites. They've been around forever.

Joe

[fornax (Phil)]

I use SkyMap on my android phone, and it always gets me pretty damn close

[AstralTraveller (David)]

I have a nice Suunto mirror compass with built in mag dec adjustment. I simply sight up the RA axis from 4-6m away using one eye. There are some bolts that I get symmetrical either side of the RA axis to know that I am looking straight down the axis. That does nicely for visual work. The other night I was observing at 310x and didn't have any problems with drift.


Yes, Silva and Suunto are good brands but if you want industrial strength compasses (full steel body) with an inclinometer you want a Brunton . It's what geologist use for strike and dip measurements. Having said that, recently some students have been using phone apps and getting to within 1-2 degrees of the Brunton readings.

[Merlin66 (Ken)]

Markus,
If you can view the Sun at noon - I've been using SunCalc to define the meridian - used with a finder scope and Baader film I can get alignment within 0.25 deg. The altitude can be set with a cardboard gauge and spirit level.

Marking the tripod feet position for the "next" time makes things very easy and quick.
https://www.suncalc.org/

[algwat (Alan)]

Try google earth satellite image for your site. Align north.
Find horizon landmarks at distance that are north or south. Measure the difference angles and calculate back to North or south proper alignments. These can be distant lamp posts at night is necessary.

[algwat (Alan)]

Oh forgot to say, mark these on the fence or in dome for next time...8)

[Stonius (Markus)]

Thanks guys, I'll take all these suggestions on board. Obviously, my compass purchase made the weather gods angry, so giving these suggestions a go will have to wait till whenever if these clouds ever clear up.

Thanks!

Markus

[Ausrock (Chris)]

Markus,

I don't know whether you've seen this....... http://www.iceinspace.com.au/63-499-0-0-1-0.html but I found it to be excellent for aiding alignment of my tripod.

Also I have three different compasses, a Suunto which I've had since working in geology in the 80's, plus two others, I always use all three to cross check for true south.

[Startrek (Martin)]

Originally used a good quality Silva compass to find true south and ended up giving it away as it was so inaccurate

Got advice from an IIS member and now use the “Solar Noon Meridian” method ( and Stellarium) which is extremely accurate and gets me really close to true south. I have permanent lines marked on the ground to set up my mount in 2 observing locations

So easy now to get roughly polar aligned !!